Lymphocytes, in particular “B-cells” and “T-cells” are two of the major cell types involved in the immune response of humans and other animals. While B-cells are involved in the humoral aspects of the immune response and are responsible for antibody production, T-cells are involved in the cell mediated aspects of the immune response. However, these two lymphocyte classes work together via a complicated network of recognition factors, cytokines and other elements of the immune response.
Within the T-cells, there are two major cell classes, namely cytotoxic T-cells (Tc) and helper T-cells (Th). Upon activation, cytotoxic T-cells kill infected cells, while helper T-cells activate other cells, such as B-cells and macrophages. Naïve T-cells are activated to produce “armed” effector T-cells upon exposure to a specific antigen that is presented on the surface of an antigen-presenting cell (APC) in conjunction with a component of the major histocompatibility complex (MHC). The two major T-cell classes are often described based on their cell surface receptors. Tc cells are often referred to as “CD8” (“CD8+”) cells, and Th cells are often referred to as “CD4” (“CD4+”) cells. Despite their different functions, CD4+ and CD8+ cells do not work independently of each other. Indeed, it is known that CD8+ cells are often dependent upon CD4+ cells in mounting a response to an immunogen. Thus, CD8+ cells often require the activation of CD4+ cells in killing infected cells. In addition, it appears that in some cases, CD8+ cells are effective in killing infected cells, while in other cases, these cells are ineffective. However, despite recent advances in the understanding of the immune response, means are still needed for the reliable identification of CD8+ cell epitopes that are effective, as well as means to differentiate effective epitopes from ineffective ones.